Communication using cellular networks has traditionally been used for mobile phone applications for voice communication, and more recently applications that allow smartphones, tablets, computers and the like to handle data communications, such as Internet browsing and so forth.
A currently popular vision of the future development of communication using cellular networks is the possibility of having huge numbers of small autonomous devices which typically transmit and receive only small amounts of data infrequently (for example once per week to once per minute), or which are polled for data. These devices are assumed not to be associated with humans, but are rather sensors or actuators of different kinds, which communicate with application servers that configure the devices and receive data from them, within or outside the cellular network. Hence, this type of communication is often referred to as machine-to-machine (M2M) communication, and the devices may be denoted machine devices (MDs). In the Third Generation Partnership Project (3GPP) standardization, the corresponding alternative terms are machine type communication (MTC) and machine type communication devices (MTC devices), with the latter being a subset of the more general term user equipment (UE).
Due to the nature of MTC devices and their assumed typical applications, it follows that they will often have to be very energy efficient, since external power supplies will often not be available, and since it is neither practically nor economically feasible to frequently replace or recharge their batteries. In some scenarios the MTC devices may not even be battery powered, but may instead rely on energy harvesting, i.e. gathering energy from the environment, opportunistically utilizing (the often very limited) energy that may be tapped from sunlight, temperature gradients, vibrations, and so on.
For such energy deprived devices whose traffic is characterized by small and infrequent transactions (often delay tolerant), it is important to minimize their energy consumption.
During the time periods between communication events the devices consume energy, for example by keeping the radio receiver active to monitor transmissions from the cellular network. Since the periods between the communication events are far longer than the actual communication events, this energy consumption represents a significant part of the overall energy consumption, and may even dominate the energy consumption in scenarios where the communication events are very infrequent.
During the communication events the actual uplink (UL) transmissions naturally consume significant amounts of energy. This is magnified by the large control signalling overhead that may be associated with a communication event.
A mechanism that has been introduced in cellular networks in order to save energy in user equipment devices, for example between communication events, is the discontinuous reception (DRX) mode of operation. The discontinuous reception mode allows a user equipment device to remain in an energy-saving sleep state most of the time, while waking up to listen for pages in idle mode DRX, or downlink resource assignments (i.e. downlink transmissions) in connected mode DRX.
FIG. 1a shows the idle and connected modes of a communication system, for example the RCC_IDLE and RCC_CONNECTED states of the Radio Resource Control (RRC) connection states, or the EMM-IDLE and EMM-CONNECTED states of the Evolved Packet System (EPS) Mobility Management (EMM) connection states. During the RRC_IDLE or EMM-IDLE states a user device listens to paging messages (at rare occasions) and is otherwise in a DRX sleep mode, i.e. energy saving mode. During the RCC_CONNECTED or EMM-CONNECTED states the user device is known on a cell level, but does not necessarily have an uplink grant or a downlink assignment. The user device may have DRX settings that are specific to that user device. FIG. 1b shows how various timers, such as inactivity timers, on-duration timers, retransmission timers, are used during DRX cycles in an RRC_CONNECTED state.
In order to make DRX mechanisms even more effective for energy deprived MTC devices, 3GPP is working on extending the maximum DRX cycle length, and thus the sleep period, both for idle mode DRX and connected mode DRX. Therefore, a DRX cycle essentially consists of a sleep period followed by an active period and this cycle is repeated over and over again until the device is detached from the network. Typically, but not necessarily, the sleep period is longer than the active period. A DRX cycle may have a more complex structure than described above, but for the purpose of this patent application, the simplified DRX cycle description suffices.
The idle mode DRX cycle, i.e. the paging cycle, is configured in the user equipment device through parameters in the system information (SI) that is broadcast in each cell, in conjunction with UE specific parameters in the form of International Mobile Subscriber Identity, IMSI, modulo 1024 and an optional UE specific DRX cycle length. Alternatively, it is also possible to configure a UE specific paging cycle. The connected mode DRX cycle and other DRX parameters (when used) are configured in the UE through optional parameters typically in a RRCConnectionReconfiguration message, during the idle to connected mode transition or later during the connected mode.
Although DRX provides power saving capabilities, UE or MTC devices having communication events which are short and often infrequent suffer from the disadvantage that power consumption is still a problem, especially since an infrequently communicating MTC device (or other UE) will go through a transition from an idle mode to a connected mode prior to every communication event. FIG. 2 illustrates the extensive signalling procedure involved during a typical transition from an idle mode to a connected mode (further details of which will be described later). This signalling during a transition from one mode to another can contribute a significant factor to the overall energy consumption of such devices.